1. Field of the Invention
The present invention relates to a method of refining high purity titanium.
2. Description of the Background
With the rapidly increasing degree of integration of LSI in recent years, the electrode materials used for LSI must, necessarily, have higher purity and strength. For example, it is now necessary to rescue the delay of signal transmission due to finer electrode wiring and metal materials are now in use having lower resistances, higher purities and higher melting points than polysilicon which was used previously.
High purity and high melting point metal materials which are used as LSI electrodes are molybdenum, tungsten and titanium or their silicides, for example. Titanium is particularly desirable due to its excellent specific strength, workability and corrosion resistance.
However for titanium to be used as an electrode material for semiconductors, a high purity is indispensable. As a refining method for obtaining high purity titanium, the iodization process is conventionally used. A conventional refining method of high purity titanium by iodization will now be described in conjunction with FIG. 2.
A reactor 1 is housed in an electric furnace 2, with a filament 3 inserted at its axial center. Inside the reactor 1, crude titanium 4 is held, surrounding the filament 3. As iodine in an iodine container 6 is led into the reactor 1, while heating the filament 3 by supplying power from a source 5, after evacuating the inside of the reactor 1, the following reactions will take place in the hermetically closed reactor 1: EQU Crude Ti+2I.sub.2 .fwdarw.TiI.sub.4 (Synthetic reaction) EQU TiI.sub.4 .fwdarw.High purity Ti+2I.sub.2 (Thermal decomposition reaction)
The synthetic reaction of crude titanium with iodine proceeds inside the reactor in which the crude titanium is held on its perimeter, with the reaction temperature at 200.degree.-400.degree. C. The thermal decomposition reaction of the tetraiodide obtained by the synthesis of crude titanium and iodine proceeds on the filament at the axial center of the reactor, with the reaction temperature at 1,300.degree.-1,500.degree. C. The iodine produced as a byproduct by the thermal decomposition of titanium tetraiodide diffuses inside the reactor to its perimeter, to be cyclically used for the synthetic reaction of crude titanium with iodine. Thus with regard to the above-mentioned reactions, deposition of high purity titanium is continued, with the perimeter inside the reactor providing the low temperature zone, and its axial center the high temperature zone.
Unfortunately, the above-described process is attended by three serious drawbacks, each of which will now be discussed.
First, since titanium tetraiodide is used as the reactant gas for the thermal decomposition reaction which proceeds on the filament located at the axial center of the reactor, the reaction temperature is extremely high, i.e., 1,300.degree.-1,500.degree. C. At such a high temperature, there is the possibility that any metal impurities contained in the reactant gas might undergo thermal decomposition, for them to be mixed into the deposited titanium. This prevents the attainment of a higher purity for the deposited titanium.
Second, since the synthetic reaction of titanium tetraiodide proceeds inside the reactor on the perimeter thereof at the relatively low temperature of 200.degree.-400.degree. C., for example, the high m.p. lower valent titanium iodides (TiI.sub.2 and TiI.sub.3) tend to form as byproducts in the solid state. The lower valent titanium iodides which have formed in the solid state coat the crude titanium surface, impeding the synthetic reaction, thus hindering the continuation of the reactions.
Third, as the reaction proceeds inside of the reactor, the metal impurities in the crude titanium are concentrated on the crude titanium surface or in the iodide gases. On this account, the purity of the deposited titanium gradually decreases with the progress of the reaction
Thus, a need continues to exist for a method of refining high purity titanium whereby the formation of solid byproducts is suppressed, while preventing the contamination of gases inside the reactor by the metal impurities in the crude titanium and inhibiting thermal decomposition of metal impurities by lowering the thermal decomposition temperature for titanium decomposition.